Single phase power question.

[RavenJoe]

I have been asked a particular question manys times and have had a hard time explaining it. So maybe some of you bright gentlemen or ladies can help me better articulate my answer. In a single phase 240 volt load. I am using a 2 pole 30 amp breaker to supply the load. Now, if I have 30 amps per pole. Do I have 60 amps total to supply the load ? Single phase 240 volt power is hard to explain in comparison 120 volt power. One hot leg and a neutral. As opposed to 2 hot legs in 240. Thanks for any replies.

 

[gordonl]

The 30A will circulate phase to phase with no neutral current, so the wires only see 30A. But the power to the load is doubled versus a 30A 120V load because it is a 30A 240V load.

This is the same if you have two lights connected phase to neutral from the two seperate hot wires, although they are connected phase to neutral at the same point on the neutral there would be no neutral current because the two currents would cancel each other out at the neutral point because they are opposite to each other. (120/240V system) The two phases in a 120/240V system are direct opposites.

 

[electricpete]

Another way of restating Gord's answer(I think) might be:

You do not gain any current carrying capability since the two poles are in series, each is still limited to 30A and total circuit current is limited to 30A.

But you can get more work out of 30A at 240v then you can out of 30A at 120v (assuming you select an appliance rated for the voltage you are using).

 

[230842]

I'd dare to make another suggestion: It is theoretically and practically reasonable to consider 240 V supply as two direct current, (even if AC current) voltage sources 120 V each, connected in series, the common point being the neutral point connected to ground. So, it is possible to have loads fed at 120 V or 240 V whith a maximum 30 A current per source. This analogy could help you to devise a more simple answer. I hope... Julian

 

[SteveKW]

Electripete has it right...It's a SERIES circuit and that's all.
On a 240 supply you have two 120 volt transformers in series adding to 240, with a neutral tapped in the center. With a 240 volt load the current passes through both transformers, BOTH breakers and the load to form a SERIES LOOP. In series all the currents are the same, 30 in, 30 out, 30 in, etc.

Now when you have a 120 volt load its ONLY in series with ONE 120 volt TRANSFORMER and the current flows in a single series loop with that ONE transformer. A series loop of one breaker one transformer and one load.

When you have a combination load of 240 & 120 everything still stays in series with its "matched" transformer but the 120 volt load ADDs to the current of that one particular transformer and breaker. Think of it like a 240 volt load being a single lane one way road. A combination load of 240 and 120 as being a single lane road but where the 120 volt load is ... another lane has been added so the current can take a different path at the neutral to complete its own particular series loop!

 

[RavenJoe]

Thanks guys you have been a big help.